1. Field of the Invention
This invention relates generally to electrode materials for lithium ion batteries, and, more specifically, to lithium alloy cermet materials and methods for making same.
2. Description of the Related Art
Lithium metal anodes are unworkable in rechargeable batteries containing liquid electrolytes due to dendrite formation, which leads to performance degradation and unsafe conditions. Despite these problems, lithium is an attractive anode material due to its high energy density. Incorporation of lithium metal into electrodes or other devices is difficult because it is a soft, sticky material, with the consistency of chewing gum. It is sometimes mixed with ceramic powders (with considerable difficulty) to make it easier to handle, but this does not significantly increase its surface area. It would be useful to have lithium in a form that has very high surface area in order to maximize the power density. Although some success has been achieved with Li metal electrodes and high temperature solid polymer electrolytes, these batteries are not acceptable for many applications. Current electrodes use lithium stored in graphite intercalation electrodes or in complex alloys containing tin or silicon.
Because of their very large theoretical capacities, lithium alloys such as Li—Sn and Li—Si are attractive candidates to replace graphite as electrode material in lithium ion batteries. In practice, however, the binary materials suffer from significant irreversible capacities, poor cyclability, and questionable rate capability. The problems arise from the complexity of the phase diagrams and the fact that substantial atomic rearrangements and large volume changes accompany a series of first-order phase transitions. Thus fresh surfaces are produced and exposed to the electrolyte, causing disconnection and isolation of active material, and inhomogeneity within the composite electrode.
An electrode in which a solid solution of lithium and another element can exist over a wide composition range would be highly advantageous. Then charging and discharging could result in continuous and relatively stress-free volume changes similar to those in a well-behaved lithium foil electrode, thus reducing irreversible capacity losses and electrolyte consumption. More importantly, a solid solution exhibits a sloping potential vs. composition profile, which reflects the presence of a driving force for relaxation to a uniform composition. Given sufficient mobility in the alloy, lithium would be transported away from the surface during deposition, countering the tendency to faun dendrites.
There are few examples of solid solutions of lithium and another element that can exist over a wide composition range and also have significant capacities. The Li—Sn (lithium-tin) and Li—Si (lithium-silicon) systems have good capacities but remain as solid solutions over only very limited composition ranges. There is a single phase over a range from 38% to 62% lithium in the Li—Hg (lithium-mercury) system, but its capacity is low at only 129 mAh/g. There is a single phase over even smaller ranges for the Li—In (lithium-indium) and Li—Cd (lithium-cadmium) systems. The most attractive candidate is the Li—Mg (lithium-magnesium) system, which has a solid solution with the body centered cubic (bcc) lithium structure over the entire range from 30% to nearly 100% lithium. In the range from 40% to 70% Li, for example, the Li—Mg solid solution has a capacity of 1100 mAh/g.
Li—Mg alloy anodes have been described previously by R. T. Mead in U.S. Pat. No. 3,895,962 (1975); G.-M. Raynaud, G. Regazzoni, G. Nussbaum, and M. Reboul in U.S. Pat. No. 5,102,475 (1992); S. F. Hope and J. B. Kejha in U.S. Pat. No. 5,350,647 (1994); and D. O. Hobson in U.S. Pat. No. 5,705,293 (1998). Although the Li—Mg system is known to have high mobility for lithium in the solution phase, Li—Mg alloys have not been used in manufacturing rechargeable batteries.
What are needed are materials that can use Li-M (lithium-metal) alloys with wide ranges of solid solubility and high capacities in electrodes with high surface areas, thereby enhancing power performance and reducing macroscopic volume changes.